The present invention relates to a semiconductor technology, more particularly to an epitaxial growth system for a surface emitting laser using a real time laser reflectometry apparatus and a method for manufacturing a surface emitting laser using it.
The growth of electron and photon structure related epitaxial layer over GaAs or InP substrates using MOCVD method, which is a method for growing semiconductor hetero-thin-film, has been widely studied up to now, and it is also prospected to be studied from now on in order to develop a more improved element.
VCSEL(Vertical-Cavity Surface Emitting Laser) is a semiconductor laser developed in the latter part of 1980""s, which is manufactured based on a new concept.
Other conventional lasers are a edge-emitting laser, in which the laser beam is emitted from the cross section of semiconductor substrate. In contrast, VCSEL is a surface emitting laser, in which the laser beam is emitted from the surface of semiconductor substrate. The VCSEL has many advantages for example, low threshold current, high integration capability and high power in array type, so that it is spotlighted as the next-generation light source. The quality of such VCSEL device depends on the uniformity of thickness of DBR (Distributed Bragg Reflector) which is the basic structure therefor. The entire thickness of VCSEL structure is much higher than that of the conventional edge-emitting laser due to the characteristics of the DBR(Distributed Bragg Reflector) structure, so that the growth thereof may be in trouble. For example, the long-time growth required for VCSEL may cause some problems in uniformity and reproducibility of the thickness of epitaxially grown layers, which are the most challenging problems being studied for the VCSEL structure.
According to one conventional method for growing VCSEL structure, multiple material layers used for composing a DBR epi-layer and a cavity epi-layer are formed separately by epitaxial growing method and then the thickness of the material layers are measured by an electron microscope so as to estimate the epitaxially growing speed of respective layers. Upon the estimated growing speed, the time durations required for forming the desired thickness of the material layers within the DBR and the cavity layers are determined. This method has a problem in that the weak reproducibility of the growing equipment may make it impossible to obtain a designed uniform epi-layers since the conditions of growth equipment are not typically maintained uniform for a long time.
In accordance with another conventional technology, the reproducibility of the growing thickness have been more or less improved by a real time laser reflectometry. In other words, the development of a real time laser reflectometry make it possible to know the growing speeds of epi-layers in-situ fabrication even though the unstableness of the environment within the growing equipment is more or less weak. Specifically, the growing time duration of a DBR layer can be controlled in real time so as to obtain the desired thickness of DBR layer as it has been designed. However, in order to use this method the index of refraction of the layer being grown should be known in advance. This is because the index of refraction of epi-layer is required for calculating a growing speed in real time. Thus, there is a problem in that an additive and interruptive step for measuring a index of refraction of the material used in epitaxial growing, must be undesirably required for measuring a growing speed, if the index of refraction of the material is not known in advance.
Accordingly, the object of the present invention is to provide an epitaxial growing system of surface emitting laser and a method for manufacturing it, in which the reproducibility of the epi-layers in thickness can be achieved without any information on the index of refraction and/or the growing speed of epitaxial material layers in advance.
To achieve the above object, the present invention uses a measuring laser beam in laser reflectometry, the wavelength of which is substantially the same with the wavelength of VCSEL and a reflected wavelength of DBR, for eliminating the need of knowledge of index of refraction and growing speed in advance. If the same wavelength is selected, the middle step required for physical property analysis can be omitted. Also, two laser, one is main and the other is subsidiary, may be used for improving precision, in order to eliminate the need of epitaxially growing the buffer layer. The laser reflectometry can calculate the growing speed of epitaxial layer in situ-fabrication by using a pre-calculated period of a reflected signal from the buffer, if the apparatus is given in advance with the index of refraction of the epitaxial layer. The DBR structure is a designed structure so that the reflectance thereof in a special wavelength is nearly set to 1. The VCSEL structure includes a cavity layer between such lower and upper DBR structures.
The present invention is devised based on that the wavelength of the laser used for measurement are selected to be the same with those of DBR and VCSEL so as to perform a real time epitaxial growth without the need of pre-knowledge of the index of refraction thereof. The two lasers having different wavelengths are used so as to perform a real time epitaxial growth of an unknown refractive index material of DBR without any buffer layer grown.
To achieve the object of the present invention, there is provided an epitaxial growing system for a surface emitting laser including a plurality of material layers, with a reactor for epitaxial growth, comprising: a measuring laser for applying a laser beam having the same wavelength with that of the surface emitting laser to a semiconductor structure being grown in the reactor, the semiconductor structure being fabricated to be the surface emitting laser; a detector for detecting a reflected signal of the measuring laser beam applied to the semiconductor structure; means for estimating at least one period each of which is the time duration required for growing a specific thickness of one of the material layers by performing an analysis of the reflected signal from the detector; and means for controlling growth time durations of respective material layer based on the result of the means for estimating.
In one preferred embodiment, the epitaxial growing system further includes an analog to digital converter for receiving an output of the detector and the measuring laser is substantially composed of a diode laser having a wavelength of 1.5 xcexcm. Also, the detector may be a Ge detector.
According to another preferred embodiment, there is provided an epitaxial growing system for a surface emitting laser including a plurality of material layers, with an reactor for epitaxial growth, comprising: a main measuring laser for applying a first laser beam to a semiconductor structure in the reactor for measuring a reflectance of the semiconductor structure, wherein the semiconductor structure is fabricated to be the surface emitting laser, and a wavelength of the first laser beam is substantially the same with a wavelength of the surface emitting laser; a subsidiary measuring laser for simultaneously applying a second laser beam with the first laser beam to the semiconductor structure; a first detector for detecting a reflected signal of the main measuring laser; a second detector for detecting a reflected signal of the second laser beam; means for estimating at least one period each of which is the time duration required for growing a specific thickness of one of the material layers by performing an analysis of the reflected signal of the main measuring laser from the first detector and the reflected signal of the subsidiary measuring laser from the second detector; and means for controlling growth time durations of respective material layer based on the result of the means for estimating.
In a specific embodiment, the main measuring laser is substantially composed of a diode laser having a wavelength of 1.5 xcexcm and the subsidiary measuring laser is a He-Ne laser having a wavelength of substantially 0.633 xcexcm. Also, the first detector is a Ge detector and the second detector is a Si detector.
In accordance with another aspect of the present invention, there is provided a method for manufacturing a surface emitting laser, comprising the steps of: applying a main measuring laser beam to a semiconductor structure using a main measuring laser, wherein the semiconductor structure is fabricated to be the surface emitting laser and a wavelength of the main measuring laser is substantially the same with a wavelength of the surface emitting laser; detecting a reflected signal of the main measuring laser beam; estimating at least one period required for growing a specific thickness of a material layer by performing an analysis of the reflected signal; and controlling growth time durations of material layers being subsequently grown, based on the result of the step of estimating.
In one preferred embodiment of the present invention, the surface emitting laser comprising the lower Bragg reflective layer, the cavity layer and the upper Bragg reflective layer. In addition, the surface emitting laser may further comprise a buffer layer under the lower Bragg reflective layer. Specifically, the buffer layer is composed of InAlGaAs/InAlAs layer; each of the upper and the lower Bragg reflective layers is composed of a plurality of InAlGaAs/InAlAs layers; the cavity layer is composed of InAlGaAs/InAlAs layer; when the wavelength of the surface emitting laser is xcex, the thicknesses of respective InAlGaAs layer and respective InAlAs layer included in the upper and the lower Bragg reflective layers are xcex/4 and the thicknesses of the InAlGaAs layer and the InAlAs layer within the cavity layer are xcex/2.
It is preferable that the laser beam of the main measuring laser is applied with an incident angle so that the following equation is satisfied, in order that an effective indexes of refraction at a growth temperature are substantially the same with those at a room temperature,
N2=n2xe2x88x92(sin xcex8)2
wherein xe2x80x98Nxe2x80x99 represents the effective indexes of refraction at the growth temperature, xe2x80x98nxe2x80x99 represents a low temperature index of refraction and xe2x80x98xcex8xe2x80x99 represents the incident angle of the laser beam.
In another preferred embodiment, the method further comprises the steps of: simultaneously applying a subsidiary measuring laser beam to the semiconductor structure with the main measuring laser beam by using a subsidiary measuring laser having different wavelength from that of the main measuring laser; detecting a reflected signal of the subsidiary measuring laser beam; and analyzing the reflected signal of the subsidiary measuring laser beam, wherein the steps of estimating the period and controlling the growth time durations are performed with reference to the result of the reflected signal of the subsidiary measuring laser beam. Here, the main measuring laser may have substantially 1.5 xcexcm of wavelength and the subsidiary measuring laser may have substantially 0.633 xcexcm of wavelength.
According to another embodiment, there is provided a method for manufacturing a surface emitting laser having a first wavelength and including a plurality of epitaxial growing layers, comprising the steps of: growing a buffer layer composed of a first material layer and a second material layer, while measuring a reflectance of a semiconductor structure being grown using a measuring laser having a second wavelength same with the first wavelength, the semiconductor structure including the buffer layer and being to become the surface emitting laser; producing a first period and a second period by performing an analysis for the measured reflectance during the buffer layer being grown, wherein the first period is a time duration required for growing a predetermined thickness of the first material layer and the second period is a time duration required for growing a predetermined thickness of the second material layer; and controlling a growing time for the plurality of epitaxial layers, based on the first period and the second period.
According to still another preferred embodiment of the present invention, there is provided a method for manufacturing a surface emitting laser having a first wavelength and including a plurality of epitaxial layers, each of which includes at least one of a plurality of first material layers and a plurality of second material layers, comprising the steps of: growing one of the first material layer, while continuously measuring reflectances thereof by simultaneously using a main measuring laser and a subsidiary measuring laser, wherein the main measuring laser has a second wavelength same with the first wavelength and the a subsidiary measuring laser has a third wavelength different from the first wavelength; stopping the growth of the first material layer and estimating a first period and a maximum point subsidiary reflectance, when the reflectance of the main measuring laser is firstly maximized, wherein the first period is the time duration required for growing a predetermined thickness of the first material layer and the maximum point subsidiary reflectance is the reflectance of the a subsidiary measuring laser when the reflectance of the main measuring laser is firstly maximized; subsequently growing one of the second material layer, while continuously measuring reflectances thereof by simultaneously using a main measuring laser and a subsidiary measuring laser; stopping the growth of the second material layer and estimating a second period and a minimum point subsidiary reflectance, when the reflectance of the main measuring laser is firstly minimized, wherein the second period is the time duration required for growing a predetermined thickness of the second material layer and the minimum point subsidiary reflectance is the reflectance of the a subsidiary measuring laser when the reflectance of the main measuring laser is firstly maximized; and controlling growing time durations for the plurality of epitaxial layers, based on the first period, the second period, the maximum point subsidiary reflectance and the minimum point subsidiary reflectance.